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Stellar population
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===Population III stars=== {{anchor|Population III stars}} [[File:Ssc2005-22a1.jpg|thumb|right|upright=1.4|Possible glow of population III stars imaged by [[NASA]]'s [[Spitzer Space Telescope]]]] Population III stars<ref name=Tominga-etal-2007/> are a hypothetical population of extremely massive, luminous and hot stars with virtually no [[metallicity|"metals"]], except possibly for intermixing ejecta from other nearby, early population III supernovae. The term was first introduced by Neville J. Woolf in 1965.<ref>{{cite journal |last1=Green |first1=Louis |title=Observational Aspects of Cosmology |journal=Sky and Telescope |date=April 1966 |volume=31 |page=199 |bibcode=1966S&T....31..199G |url=https://ui.adsabs.harvard.edu/abs/1966S%26T....31..199G/abstract}}</ref><ref>{{cite journal |last1=Thornton |first1=Page |title=Observational Aspects of Cosmology |journal=Science |date=March 1966 |volume=151 |issue=3716 |pages=1411-1414,1416-1418 |doi=10.1126/science.151.3716.1411 |pmid=17817304 |bibcode=1966Sci...151.1411P |url=https://ui.adsabs.harvard.edu/abs/1966Sci...151.1411P/abstract}}</ref> Such stars are likely to have existed in the very early universe (i.e., at high redshift) and may have started the production of [[chemical element]]s heavier than [[hydrogen]], which are needed for the later formation of [[planet]]s and [[life]] as we know it.<ref name=Sobral-etal-2015-06/><ref name=NYT-2015-06-17/> The existence of population III stars is inferred from [[physical cosmology]], but they have not yet been observed directly. Indirect evidence for their existence has been found in a [[gravitationally lensed galaxy]] in a very distant part of the universe.<ref name=Fosbury-etal-2003/> Their existence may account for the fact that heavy elements β which could not have been created in the [[Big Bang]] β are observed in [[quasar]] [[emission spectrum|emission spectra]].<ref name=Heger-Woosley-2002/> They are also thought to be components of [[faint blue galaxy|faint blue galaxies]]. These stars likely triggered the universe's period of [[reionization]], a major [[phase transition]] of the hydrogen gas composing most of the interstellar medium. Observations of the galaxy [[UDFy-38135539]] suggest that it may have played a role in this reionization process. The [[European Southern Observatory]] discovered a bright pocket of early population stars in the very bright galaxy [[Cosmos Redshift 7]] from the reionization period around 800 million years after the Big Bang, at {{math|''z'' {{=}} 6.60}}. The rest of the galaxy has some later redder population II stars.<ref name=Sobral-etal-2015-06/><ref name=Astronomy-2017-06-17-ESO/> Some theories hold that there were two generations of population III stars.<ref name=Bromm/> [[File:NASA-WMAP-first-stars.jpg|thumb|right|upright=1.3|Artist's impression of the first stars, 400 million years after the [[Big Bang]]]] Current theory is divided on whether the first stars were very massive or not. One possibility is that these stars were much larger than current stars: several hundred [[solar mass]]es, and possibly up to 1,000 solar masses. Such stars would be very short-lived and last only 2β5 million years.<ref name=Ohkubo-etal-2009/> Such large stars may have been possible due to the lack of heavy elements and a much warmer [[interstellar medium]] from the Big Bang.{{Citation needed|date=June 2017}} Conversely, theories proposed in 2009 and 2011 suggest that the first star groups might have consisted of a massive star surrounded by several smaller stars.<ref name=Space_FS/><ref name=Space_MS/><ref name=Carr-essay/> The smaller stars, if they remained in the birth cluster, would accumulate more gas and could not survive to the present day, but a 2017 study concluded that if a star of 0.8 solar masses ({{solar mass}}) or less was ejected from its birth cluster before it accumulated more mass, it could survive to the present day, possibly even in our Milky Way galaxy.<ref name=Dutta-etal-2017/> Analysis of data of extremely low-[[metallicity]] population II stars such as [[HE 0107-5240]], which are thought to contain the metals produced by population III stars, suggest that these metal-free stars had masses of 20~130 solar masses.<ref name=Umeda-Nomoto-2003/> On the other hand, analysis of [[globular cluster]]s associated with [[elliptical galaxies]] suggests [[pair-instability supernova]]e, which are typically associated with very massive stars, were responsible for their [[metallicity|metallic]] composition.<ref name=Puzia-etal-2006/> This also explains why there have been no low-mass stars with zero [[metallicity]] observed, despite models constructed for smaller population III stars.<ref name=Siess-Livio-Lattanzio-2002/><ref name=gibson2012/> Clusters containing zero-metallicity [[red dwarfs]] or [[brown dwarfs]] (possibly created by pair-instability supernovae<ref name=Salvaterra-Ferrara-Schneider-2004/>) have been proposed as [[dark matter]] candidates,<ref name=Kerins-1997/><ref name=Sanchez-1997/> but searches for these types of [[Massive compact halo object|MACHO]]s through [[gravitational microlensing]] have produced negative results.{{Citation needed|date=February 2019}} Population III stars are considered seeds of black holes in the early universe. Unlike high-mass [[black hole]] seeds, such as [[Direct collapse black hole|direct collapse black holes]], they would have produced light ones. If they could have grown to larger than expected masses, then they could have been [[Quasi-star|quasi-stars]], other hypothetical seeds of heavy black holes which would have existed in the early development of the Universe before hydrogen and helium were contaminated by heavier elements. Detection of population III stars is a goal of NASA's [[James Webb Space Telescope]].<ref name=JW_PIII/> On 8 December 2022, astronomers reported the possible detection of Population III stars, in a high-[[redshift]] galaxy called RX J2129βz8He II.<ref name="ARX-20221208">{{cite arXiv |author=Wang, Xin |display-authors=et al. |title=A strong He II Ξ»1640 emitter with extremely blue UV spectral slope at z=8.16: presence of Pop III stars? |date=8 December 2022 |class=astro-ph.GA |eprint=2212.04476 }}</ref><ref name="QUANT-20230130">{{cite news |last=Callaghan |first=Jonathan |title=Astronomers Say They Have Spotted the Universe's First Stars - Theory has it that "Population III" stars brought light to the cosmos. The James Webb Space Telescope may have just glimpsed them. |url=https://www.quantamagazine.org/astronomers-say-they-have-spotted-the-universes-first-stars-20230130/ |date=30 January 2023 |work=[[Quanta Magazine]] |accessdate=31 January 2023 }}</ref>
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